This invention relates generally to milling cutters and more particularly to a ball nose end mill having a single replaceable cutting insert retained between clamping jaws, for use in high precision milling operations.
Milling cutters are commonly used for profile and cavity milling in the auto, aircraft, die and mold industries. In order to achieve high accuracy and good surface quality in the work products produced by the milling cutters to which the present invention relates, the cutting insert should be retained in the milling cutter with a high degree of locational precision.
A problem with prior art milling cutters utilizing replaceable cutting inserts is that the latter are either flat plate-like or generally prismatic in shape and are clamped between two clamping jaws having clamping surfaces with geometries complementary to those of the surfaces of the cutting insert being clamped, i.e., flat or prismatic, respectively. The cutting insert is secured in position by means of a clamping screw that passes through through bores in the clamping jaws and in the cutting insert. Due to manufacturing tolerances it is difficult to ensure that the cutting insert is on a true centerline when retained in the milling cutter. Such manufacturing tolerances include, for example, the diameter of the clamping screw, the diameter of the through bores and the location of the through bores. That is, for cutting tools such as end mills having a single replaceable cutting insert retained between clamping jaws the radial location of two diametrically opposite cutting edges of the cutting insert can be ascertained only to within the total manufacturing tolerances. This being the case, the points on the cutting edges of the cutting insert will not necessarily lie precisely on a flight circle concentric with the axis of rotation of the milling cutter. For milling cutters having flat plate-like cutting inserts, manufacturing tolerances of the clamping screw and the through bore in the milling cutter and in the cutting insert are the main contributing factors to the radial location imprecision of the cutting insert. The radial location precision can be improved by using inserts that have generally prismatic abutting surfaces clamped in clamping jaws having complementary shaped clamping surfaces. However, even in this case radial location imprecision of the cutting insert will exist. The main contribution to the radial location imprecision coming from geometrical factors such as the tolerance of manufacture of the angle between adjacent surfaces comprising the prismatic abutting surfaces.
In EP 0 417 862 B1 a cutting tool assembly is described which ensures effective clamping of a cutting insert in an insert holder so that the cutting insert is effectively retained against displacement under cutting forces, and so that the relative position of the cutting insert with respect to the cutting insert holder is maintained substantially invariant, both during cutting operations and also when inserts have to be replaced. In the embodiment shown in FIG. 3 of ""862, effective retention of the cutting insert is obtained between ribs of the clamping jaws of the cutting insert holder and the cutting insert along discrete axially directed locations which are symmetrically disposed with respect to, and spaced from, a longitudinally directed median plane. In the embodiment shown in FIG. 4 of ""862, instead of effective retention along discrete axially directed locations, the retention is along narrow axially directed intermediate curved portions.
In both the embodiments described in ""862 the ribs of the clamping jaws are formed with a clamping surface of substantially circular cross-sectional shape and the clamping contact between the clamping surfaces of the clamping jaws and the cutting insert is designed to take place along predetermined axial locations by forming the clamping surfaces of the cutting insert with side curved portions having a larger radius of curvature than that of the clamping surface of the clamping jaw and a central curved portion have a smaller radius of curvature than that of the clamping surface of the clamping jaw.
It should be noted that in both embodiments described in EP ""862 the clamping surfaces of the cutting insert are of a non-uniform cross section and that in both embodiments the side portions of the clamping surfaces of the cutting inserts do not abut the clamping surfaces of the clamping jaws. A disadvantage of such a clamping arrangement is that it cannot be applied to cutting tool assemblies such as end mills comprising a single replaceable cutting insert retained between a pair of clamping jaws. The cutting inserts of such cutting tool assemblies have a width, measured between the side cutting edges, which is much greater than their thickness, measured between the clamping surfaces of the cutting insert, and the clamping arrangement described in EP ""862 would not provide adequate support for the side cutting edges of the cutting insert.
It is therefore an object of the present invention to overcome the aforementioned disadvantage and to provide a milling cutter that will ensure accurate radial location of replaceable cutting inserts when clamped in the milling cutter whilst providing adequate support for the side cutting edges of the cutting insert.
It is a further object of the present invention to provide an indexable cutting insert for such a milling cutter.
In accordance with the present invention, there is provided a cutting tool assembly comprising a cutting insert holder, a cutting insert and a clamping screw, the cutting insert holder comprising a clamping portion connected to a body portion;
the clamping portion comprising:
a lower clamping jaw having a lower peripheral surface and an upper clamping surface;
an upper clamping jaw resiliently connected to the lower clamping jaw, the upper clamping jaw having a lower clamping surface and an upper peripheral surface;
a through bore passing through the upper and lower clamping jaws, the through bore being threaded in one of the upper or lower clamping jaws;
an insert receiving slot defined between the upper and lower clamping surfaces;
the cutting insert comprising:
an insert upper clamping surface bounded by an upper edge;
an insert lower clamping surface bounded by a lower edge;
a peripheral side surface between the insert upper clamping surface and the insert lower clamping surface;
at least one cutting edge associated with the peripheral side surface;
at least one through hole passing through the cutting insert between the insert upper and lower clamping surfaces;
wherein:
in a front view of the cutting tool assembly, the upper and lower clamping surfaces of the lower and upper clamping jaws, respectively, are partially convex, and the insert lower and upper clamping surfaces are concave;
the cutting insert is retained in the insert receiving slot in a retained position by means of the clamping screw which passes through the through bore in the upper and lower clamping jaws of the cutting insert holder and through the at least one through hole in the cutting insert and is screwed into the threaded through bore in one of the upper or lower clamping jaws;
the lower clamping surface of the upper clamping jaw engages the insert upper clamping surface at at least one upper region of contact and the upper clamping surface of the lower clamping jaw engages the insert lower surface at at least one lower region of contact;
wherein:
the insert upper clamping surface conforms with a first cylindrical surface having a first radius of curvature, r1, and a first cylinder axis;
the insert lower clamping surface conforms with a second cylindrical surface having a second radius of curvature, r2, and a second cylinder axis;
the upper clamping surface of the lower clamping jaw partially conforms with a third cylindrical surface having a third radius of curvature, R2, and a third cylinder axis;
the lower clamping surface of the upper clamping jaw partially conforms with a fourth cylindrical surface having a fourth radius of curvature, R1, and a fourth cylinder axis;
the fourth radius of curvature, R1, is greater than the first radius of curvature, r1, the third radius of curvature, R2, is greater than the second radius of curvature, r2.
Preferably, the cutting insert holder has an axis of rotation (A) and the cutting insert has a longitudinal axis of rotational symmetry (B) and a center through which the longitudinal axis of rotational symmetry (B) passes, the longitudinal axis of rotational symmetry (B) constituting an axis of rotation of the cutting insert about which the cutting insert has 180xc2x0 rotational symmetry.
In accordance with a specific application, the first radius of curvature and the second radius of curvature are equal in magnitude, r1=r2, defining a radius of curvature, r, of the cutting insert clamping surfaces, and the third radius of curvature and the fourth radius of curvature are equal in magnitude, R1=R2, defining a radius of curvature, R, of the clamping jaws"" clamping surfaces.
In accordance with the present invention, the at least one upper region of contact comprises at least two outer regions of the insert upper clamping surface and the at least one lower region of contact comprises at least two outer regions of the insert lower clamping surface.
In accordance with one specific application, r=16 mm and R=16.2 mm.
In accordance with the present invention, the cutting insert has a width, W, which is greater than a thickness, T, where the width and thickness are measured in a front view of the cutting insert as, respectively, the distance between opposing portions of the peripheral side surface, and a maximum distance between the insert upper and lower clamping surfaces.
Generally, W/T lies in the range 2.5 to 4.0.
In accordance with a specific application, W/T is equal to 3.3.
Further in accordance with the present invention, the cutting insert has a width, W, which is greater than a thickness, t, where the width and thickness are measured in a front view of the cutting insert as, respectively, the distance between opposing portions of the peripheral side surface, and a minimum distance between the insert upper and lower clamping surfaces.
Generally, W/t lies in the range 4.0 to 6.0.
In accordance with a specific application, W/t is equal to 5.3.
Yet further in accordance with a specific application the radius of curvature, r, of the cutting insert clamping surfaces is approximately equal to W, and the radius of curvature, R, of the clamping jaw""s clamping surfaces is approximately equal to W.
Generally, the insert receiving slot has a forward end for receiving a cutting insert and a rear end at which the upper clamping jaw is resiliently connected to the lower clamping jaw.
Preferably, the rear end of the insert receiving slot is provided with at least one insert axial location surface transverse to the axis of rotation (A) of the cutting insert holder.
Still further preferably, the rear end of the insert receiving slot is provided with at least one generally cylindrical groove transverse to the axis of rotation (A) of the cutting insert holder and adjacent the at least one insert axial location surface.
Still yet further preferably, the cutting insert has two end portions adjacent the axis of rotation (B) of the cutting insert, at least one of the two end portions being provided with axial abutment surfaces transverse to the axis of rotation (B) of the cutting insert.
Preferably, the axial abutment surfaces are ground.
In accordance with a first specific application, the at least one through hole in the cutting insert is implemented as a circular bore having a bore axis located substantially at the center of the cutting insert in a top view of the cutting insert, the bore axis being perpendicular to the axis of rotation (B) of the cutting insert.
In accordance with a second specific application, the at least one through hole in the cutting insert is implemented as two circular bores, each circular bore having a bore axis and a bore diameter (D1, D2), the bore diameter of each circular bore being substantially equal in magnitude (D1=D2=D), each bore axis being located at substantially the same distance (X1=X2=X) from the center of the cutting insert in a top view of the cutting insert, the bore axes being perpendicular to the axis of rotation (B) of the cutting insert and the distance (2x) between the two bore axes being greater than the bore diameter (D).
In accordance with a third specific application of the present invention, the at least one through hole is implemented by two intersecting circular bores, each circular bore having a bore axis and a bore diameter (D1, D2), the bore diameter of each circular bore being substantially equal in magnitude (D1=D2=D), each bore axis being located at substantially the same distance from the center of the cutting insert in a top view of the cutting insert, each bore axis being perpendicular to the axis of rotation (B) of the cutting insert and the distance (Y) between the two bore axes being less than the bore diameter (D).
In accordance with the second specific application of the present invention, the cutting insert is retained in the insert receiving slot in a retained position by means of a clamping screw which passes through the through bore in the upper and lower clamping jaws of the cutting insert holder and through the circular bore in the cutting insert closest the forward end of the insert receiving slot and is screwed into the threaded through bore in one of the upper or lower clamping jaws.
In accordance with the third specific application of the present invention, the cutting insert is retained in the insert receiving slot in a retained position by means of a clamping screw which passes through the through bore in the upper and lower clamping jaws of the cutting insert holder and through the circular bore in the cutting insert closest the forward end of the insert receiving slot and is screwed into the threaded through bore in one of the upper or lower clamping jaws.
In accordance with a specific application of the invention, the peripheral side surface of the cutting insert is provided with two diametrically opposite pairs of cutting edges so that when the cutting insert is retained in the insert receiving slot, one pair of cutting edges is located closer to the forward end of the insert receiving slot and constitutes a pair of operative cutting edges and the other pair of cutting edges is located closer to the rear end of the insert receiving slot and constitutes a pair of inoperative cutting edges.
Preferably, the peripheral side surface is provided with two diametrically opposite notches which are located on a first transverse axis of rotational symmetry (T1) and which separate between the diametrically opposite sets of cutting edges, the first transverse axis of rotational symmetry (T1) passing through the peripheral side surface and perpendicular to the longitudinal axis of rotational symmetry (B), the cutting insert having 180xc2x0 rotational symmetry about the first transverse axis of rotational symmetry (T1).
Typically, each pair of cutting edges comprises one cutting edge which extends from a region close to where the longitudinal axis of rotational symmetry (B) passes through the peripheral side surface upwards towards a notch whilst the other cutting edge extends from the same region close to where the longitudinal axis of rotational symmetry passes through the peripheral side surface downwards towards the diametrically opposite notch.
Generally, each cutting edge is provided with a relief surface and a rake surface.
There is further provided in accordance with the present invention, a cutting insert having a longitudinal axis of rotation (B) and a center through which the longitudinal axis (B) passes, the longitudinal axis (B) constituting an axis of rotation of the cutting insert, the cutting insert comprising:
an insert upper clamping surface bounded by an upper edge;
an insert lower clamping surface bounded by a lower edge;
a peripheral side surface between the insert upper clamping surface and the insert lower clamping surface;
at least one cutting edge associated with the peripheral side surface;
at least one through hole passing through the cutting insert between the insert upper and lower clamping surfaces;
wherein:
the insert upper clamping surface conforms with a portion of a first cylindrical surface having a first radius of curvature, r1, and a first cylinder axis;
the insert lower clamping surface conforms with a portion of a second cylindrical surface having a second radius of curvature, r2, and a second cylinder axis.
In accordance with a specific application, the first radius of curvature and the second radius of curvature are equal in magnitude, r1=r2, defining a radius of curvature, r, of the cutting insert clamping surfaces.
Further in accordance with a specific application, longitudinal axis (B) constitutes an axis of rotational symmetry of the cutting insert about which the cutting insert has 180xc2x0 rotational symmetry.
In accordance with the present invention, the cutting insert has a width, W, which is greater than a thickness, T, where the width and thickness are measured in a front view of the cutting insert as, respectively, the distance between opposing portions of the peripheral side surface, and a maximum distance between the insert upper and lower clamping surfaces.
Generally, W/T lies in the range 2.5 to 4.0.
In accordance with a specific application W/T is equal to 3.3.
Further in accordance with the present invention, the cutting insert has a width, W, which is greater than a thickness, t, where the width and thickness are measured in an end view of the cutting insert as, respectively, the distance between opposing portions of the peripheral side surface, and a minimum distance between the insert upper and lower clamping surfaces.
Generally, W/t lies in the range 4.0 to 6.0.
In accordance with a specific application, W/t is equal to 5.3.
Still yet further preferably, the cutting insert has two end portions adjacent the axis of rotation (B) of the cutting insert, at least one of the two end portions being provided with axial abutment surfaces transverse to the axis of rotation (B) of the cutting insert.
Preferably, the axial abutment surfaces are ground.
In accordance with a first specific application, the at least one through hole in the cutting insert is implemented as a circular bore having a bore axis located substantially at the center of the cutting insert in a top view of the cutting insert, the bore axis being perpendicular to the axis of rotation (B) of the cutting insert.
In accordance with a second specific application, the at least one through hole in the cutting insert is implemented as two circular bores, each circular bore having a bore axis and a bore diameter (D1, D2), the bore diameter of each circular bore being substantially equal in magnitude (D1=D2=D), each bore axis being located at substantially the same distance (X1=X2=X) from the center of the cutting insert in a top view of the cutting insert, the bore axes being perpendicular to the axis of rotation (B) of the cutting insert and the distance (2x) between the two bore axes being greater than the bore diameter (D).
In accordance with a third specific application of the present invention, the at least one through hole is implemented by two intersecting circular bores, each circular bore having a bore axis and a bore diameter (D1, D2), the bore diameter of each circular bore being substantially equal in magnitude (D1=D2=D), each bore axis being located at substantially the same distance from the center of the cutting insert in a top view of the cutting insert, the bore axes being perpendicular to the axis of rotation (B) of the cutting insert and the distance (Y) between the two bore axes being less than the bore diameter (D).
There is yet further provided in accordance with the present invention, a cutting insert holder having an axis of rotation (A) and comprising a clamping portion connected to a body portion;
the clamping portion comprising:
a lower clamping jaw having a lower peripheral surface and an upper clamping surface;
an upper clamping jaw resiliently connected to the lower clamping jaw, the upper clamping jaw having a lower clamping surface and an upper peripheral surface;
a through bore passing through the upper and lower clamping jaws, the through bore being threaded in one of the upper or lower clamping jaws; and
an insert receiving slot defined between the upper and lower clamping surfaces;
wherein:
the upper clamping surface of the lower clamping jaw partially conforms with a portion of a third cylindrical surface having a third radius of curvature, R2, and a third cylinder axis;
the lower clamping surface of the upper clamping jaw partially conforms with a portion of a fourth cylindrical surface having a fourth radius of curvature, R1, and a fourth cylinder axis; and
in a front view of the cutting insert holder, the upper and lower clamping surfaces of the lower and upper clamping jaws, respectively, are partially convex.
In accordance with a specific application, the third radius of curvature and the fourth radius of curvature are equal in magnitude, R1=R2, defining a radius of curvature, R, of the clamping jaws"" clamping surfaces.
Generally, the insert receiving slot has a forward end for receiving a cutting insert and a rear end at which the upper clamping jaw is resiliently connected to the lower clamping jaw.
Preferably, the rear end of the insert receiving slot is provided with at least one insert axial location surface transverse to the axis of rotation (A) of the cutting insert holder.
Still further preferably, the rear end of the insert receiving slot is provided with at least one generally cylindrical groove transverse to the axis of rotation (A) of the cutting insert holder and adjacent the at least one insert axial location surface.
In accordance with one embodiment, the lower clamping surface of the upper clamping jaw is divided into three regions, two substantially identical outer regions having a first radius of curvature and a central region having a second radius of curvature and the upper clamping surface of the lower clamping jaw is divided into three regions, two substantially identical outer regions having a first radius of curvature and a central region having a second radius of curvature.
Preferably, the second radius of curvature is larger than the first radius of curvature.
There is still yet further provided in accordance with the present invention, a clamping screw having a screw axis (C) and comprising:
an upper portion;
a central portion; and
a lower portion;
the upper portion comprises an upper cylindrical portion and an upper frusto-conical portion, the upper frusto-conical portion tapers downwardly and inwardly from the upper cylindrical portion towards the central portion, the clamping screw upper portion is provided with a socket, for receiving a wrench for tightening or loosening the clamping screw, the clamping screw central portion has a central cylindrical portion connected at its upper end to the upper frusto-conical portion and at its lower end to a central frusto-conical portion, the central frusto-conical portion tapers downwardly and inwardly from the upper end thereof, to the lower end thereof, at the lower end thereof the central frusto-conical portion is connected, via a narrow neck portion, to the clamping screw lower portion, the clamping screw lower portion comprises a threaded portion of the clamping screw, the central frusto-conical portion tapers at an angle xcex8 to the screw axis (C).